High Intensity Discharge lamps are widely used in a number of commercial and industrial settings such as high bay industrial areas, lobbies and gyms among other applications. HID lamps are desirable for these applications because of their ability to provide light output with a high degree of luminous efficacy measured in lumens per watt (LPW) and an acceptable color rendering index (CRI). The arc tube of an HID lamp generally contains a fill of an ionizable gas. The fill normally constitutes a rare gas, a charge of mercury and one or more of the metal halides. During operation of a typical HID lamp, an arc discharge is established by passing a current through a pair of electrodes disposed within the arc tube containing the fill. In the arc discharge the metal halide dissociates, metal ions are thermally excited to higher energy states, and these excited atoms radiate their characteristic wavelengths. In response to the fact that such electrodes may be subject to energy loss, evaporation and chemical attack by the gas constituents of the arc tube, recent design efforts have been directed along the lines of removing the electrodes altogether.
Metal halide arc discharge lamps of this type of lamp are shown, for example, in U.S. Pat. Nos. 4,972,120; 4,810,938; 4,871,946; 4,894,589; and 4,894,590, all assigned to the assignee of the present invention and are hereby incorporated by reference. A prior art electrodeless HID lamp is described in U.S. patent application Ser. No. 07/685,371 filed on Apr. 15, 1991 for an Electrodeless High Intensity Discharge Lamp Having an Integral Quartz Outer Jacket and assigned to the same assignee as the present invention. Such an HID electrodeless lamp includes an outer envelope encasing an arc tube containing a fill of an ionizable gas capable of forming a light emitting plasma. The HID lamp further includes a solenoidal coil disposed around the outer envelope and in proximate relation to the enclosed arc tube for coupling of RF energy from an RF energy excitation source to the arc tube to ionize the fill.
As described in the previously noted U.S. patent application Ser. No. 07/685,371, the arc tube has a probe, also made of the same fused quartz material as the arc tube. The probe extends from the central portion on the surface of the upper hemisphere of the arc tube and acts as a starting aid for the lamp and as a support for the arc tube within the outer jacket. The arc tube shape is chosen so as to minimize temperature gradients around the arc tube. The outer jacket is disposed in surrounding relation to a large portion of the arc tube in such a manner as to allow for efficient thermal management of heat generated by the arc tube enclosed within the outer jacket. As described in U.S. Pat. No. 4,972,120, issued to Witting and assigned to the same assignee as the present invention, the fill contained inside the arctube generally includes volatile condensate and constituent gases which will preferably include one or more metal halides and a buffer gas which is typically an inert gas such as krypton or xenon. The fill is introduced into the arc tube through an exhaust tubing connected to the arc tube, after which it is then tipped off by heating to leave a small projection or exhaust tip-off on the surface of the arc tube in place of the exhaust tubing. The fill constituents are combined in proper weight proportions so as to achieve the desirable efficacy and color temperature characteristics of the discharge arc which will be generally toroidal in shape.
The temperature of the arc discharge ranges from a low value on the order of 900-1000K at the arc tube wall to approximately 5000K at the plasma center so that the high temperature becomes localized at the center of the discharge, there being a temperature gradient towards the walls of the arc tube and the central portion of the toroidal discharge, which are much cooler. As a consequence of this temperature gradient the arc discharge may become constricted. Another factor that contributes to the constriction of the arc discharge is chemical reactions of the metal halides that occur near the walls of the arc tube. Discussion of the chemical reactions will be mentioned in more detail hereinafter. Due to its constricted shape, the arc discharge is capable of moving around inside the arc tube since the arc tube walls are not effective in stabilizing it. Hence the arc discharge in such an electrodeless HID lamp is particularly prone to instability by virtue of the plasma having room to move around inside the arc tube. Any other factors that will contribute to the instability of the arc discharge inside the arc tube would be therefore highly undesirable. Such instability will cause the lamp to flicker and will render the lamp useless as a stable light source.
One mechanism that has been associated with the instability of the arc discharge of an electrodeless HID lamp such as that of the previously noted U.S. patent application Ser. No. 07/685,371 is movement of the condensate fill on the inside walls of the arc tube during lamp operation. In this lamp, high temperatures above 900K are required on the arc tube walls to prevent the fill material from condensing on the arc tube walls. The condensate has a natural tendency to settle around the equatorial portion of the oblate spheroidal arc tube. When a certain location on the inside surface of the arc tube is cooler than the rest of the surface, the fill material will condense at that particular location. Condensation of the fill material on the arc tube surface acts to block some light output from the arc tube walls resulting in reduced luminous efficacy of the lamp. Once condensed at the cold spot any motion of the discharge toward the cold spot will vaporize the condensate. Consequently the cold spot moves to another location of the arc tube walls where the condensate will move to. This cycle continues, causing instability of the arc discharge as the condensate moves from one cold spot location to another resulting in flickering and eventually extinction of the light source.
It would be desirable to solve the above mentioned issues relating to motion of the condensate location and light blockage from the arc tube by the condensate in order to achieve a more stable and efficacious light source. In particular it would be advantageous to eliminate the above-mentioned feedback mechanism responsible for movement of the condensate location and therefore arc discharge instability. It would also be beneficial to achieve such a stabilized light source in a manner that does not allow the condensed fill material to block a substantial amount of the light output through the arctube walls.
It is the object of this invention to provide an electrodeless arc tube with a fixed condensate location which does not change when the arc discharge location changes and to do so in a practical manner. Fixing the condensate location is intended to remove the above-described feedback mechanism partially responsible for discharge instability and to minimize the light output blocked by the condensate on the arc tube wall.